This invention relates to a process for producing high purity monoethylene glycol. Monoethylene glycol is industrially produced by hydrolysis of ethylene oxide, dewatering and purifying distillation. To improve the selectivity of the ethylene oxide (hereinafter abbreviated to EO) hydrolysis, the hydrolysis reactor is operated using a large excess of water (water:EO weight ratio=4:1 to 15:1). This makes it possible to suppress the fraction of higher glycols, especially diethylene glycol, triethylene glycol, etc. The hydrolysis reactor is customarily operated at temperatures of 120 to 250xc2x0 C. and pressures of 30-40 bar. The hydrolysis product is initially dewatered, to a residual water content of 100-200 ppm, and then separated into the various glycols in pure form.
The dewatering is generally carried out in a battery of pressure-graduated columns, with decreasing pressure. For heat integration reasons, generally only the bottoms reboiler of the first pressure column is heated with external steam, whereas all the other pressure columns are heated with the vapors from the preceding column. The feed enters each column at a point below the first plate, since no stripping section is required to separate water and glycols. Depending on the water content of the hydrolysis reactor effluent and on the pressure/temperature level of the external steam used in the first column""s bottoms reboiler, the pressure dewatering battery comprises from 2 to 7 columns. The pressure dewatering stage is followed by a vacuum dewatering stage, which generally takes place in a column equipped with a stripping section. The water obtained from the dewatering is returned to a point upstream of the hydrolysis reactor. The dewatered glycol mixture is separated into the pure materials in a plurality of columns. Monoethylene glycol, diethylene glycol and triethylene glycol are each withdrawn as top-of-column product, while all other higher glycols are obtained in the form of a mixture known as polyethylene glycols as the bottom product of the last column.
Conventional glycol plants, in addition to the product streams, customarily have only a further single outlet, the acetaldehyde purge at the bottoms reboiler of the second pressure dewatering column. There, the uncondensed fraction of the first column""s vapors used for heating is removed from the system. Thus, secondary components, either carried into the glycol plant by the water/EO stream or formed in the glycol plant as a consequence of secondary reactions, can only be removed from the system via the acetaldehyde purge or via the product streams. The latter impairs product quality and so is undesirable.
Hitherto, glycol plants were optimized only with regard to their principal functions, especially with regard to energy and capital costs reduction for the dewatering and purifying distillation. Of late, increasingly tougher requirements are being placed on the product quality of monoethylene glycol, especially with regard to the level of secondary components. There are two monoethylene glycol product qualities: technical grade (antifreeze grade) with lower purity requirements, for use as coolant, and fiber grade, with strict requirements, for use in fiber manufacture, for example. The exact specification of fiber grade varies with the customer, but for free aldehydes, reckoned as acetaldehyde, spectrophotometrically assayed as blue MBTH complex, it generally envisages the range from 7 to 20 ppm and for the minimum UV transmission it generally envisages 76%-80% at 220 nm and 90%-95% at 275 nm. The contributors to the free aldehydes measurement are in particular formaldehyde, acetaldehyde and glycolaldehyde. The UV-active substances, known as UV spoilers, are largely unknown, but are specification-destructive even in concentrations of less than 1 ppm. Examples are acrolein and crotonaldehyde.
JP-A-60,089,439 describes a process for purifying glycol by vacuum distillation with a supply of inert gas. The nitrogen stream strips out a portion of the secondary components to leave a high purity glycol which is suitable for fiber manufacture. However, the process has the disadvantage that large amounts of nitrogen are needed for effective removal of secondary components. This leads to undesirable product losses in the exit gas and to an excessively large fluid-dynamic stress on the distillation column.
DE-A-1 942 094 describes a process for purifying monoethylene glycols by steam distillation in a stripping column, the steam increasing the volatility of the impurities with regard to monoethylene glycol.
CA-C-1330350 describes a process for purifying monoethylene glycol by addition of bisulfite ions and subsequent treatment with anion exchange resins.
There are also purification processes for monoethylene glycol where the formation of secondary components is said to be reduced by special measures in the area of apparatus construction and the materials of construction used for the apparatus. DE-A-19 602 116 describes a purification process for monoethylene glycol in an apparatus whose surface has been treated with reducing phosphorus compounds.
However, the abovementioned processes have the disadvantage of requiring additives or additional equipment-based measures to recover high purity monoethylene glycol.
It is an object of the present invention to provide a simple distillative process for recovering high purity monoethylene glycol, without the use of additives or of specific materials of construction. Specification-destructive secondary components are to be removed from the system in predominantly aqueous waste streams having glycol contents of not more than 1% by weight and the secondary components in the waste streams are to be concentrated by a factor of 10-100, since too much wastewater is produced otherwise.
We have found that this object is achieved by a process for the distillative recovery of high purity monoethylene glycol from the hydrolysis product of ethylene oxide by pressure dewatering, preferably in a battery, vacuum dewatering and subsequent purifying distillation, which comprises withdrawing during the vacuum dewatering an aqueous stream which contains monoethylene glycol in a concentration below 1% by weight, preferably below 0.1% by weight, medium boilers and low boilers and which, optionally after further workup, is removed from the system.
Particular preference is given to a process in which, in addition to the abovementioned solution, the pressure dewatering takes place in a dewatering column having a stripping section with at least one separating stage, preferably with from 2 to 10 separating stages, particularly preferably with from 3 to 6 stages, and in which a portion of the overhead stream of the dewatering column(s) having a stripping section is removed from the system.
It was determined that removal of specification-destructive secondary components is particularly effective at certain locations in the process. Identifying these locations in the process is not a trivial matter, since the complex phase equilibria have hitherto made it impossible to arrive at a sufficiently confident assessment of the behavior of the secondary components. For this reason, conventional large industrial processes have only a very coarse outlet for extremely low boiling secondary components, the acetaldehyde purge at the bottoms reboiler of the second pressure dewatering column. This outlet is not optimized, since the behavior of the secondary components was largely unknown and was not taken into account at the process design stage.
The components are herein subdivided into three classes with regard to their boiling range:
1. low boilers, having a volatility greater than that of water (especially acetaldehyde, formaldehyde in pure water, acrolein),
2. medium boilers, having a volatility between that of water and monoethylene glycol (especially formaldehyde in glycol-containing aqueous solutions, formaldehyde in anhydrous monoethylene glycol, glycolaldehyde, crotonaldehyde), and
3. high boilers, having a lower volatility than monoethylene glycol (especially relatively high molecular weight aldehydes, UV spoilers).
The vacuum dewatering of the invention comprises withdrawing an aqueous stream which contains less than 1% by weight of monoethylene glycol, medium boilers and low boilers, which, optionally after further workup, is removed from the system.
The vacuum dewatering can take place in a vacuum dewatering column, in which case an aqueous stream of medium boilers and low boilers is withdrawn as a sidestream. The vacuum dewatering column is fed with a stream comprising 1-99% by weight, preferably 50-90% by weight, of monoethylene glycol, 1-99% by weight, preferably 50-10% by weight, of water and specification-destructive secondary components within the range from 1 ppm to 5%, preferably within the range from 1 ppm to 1%, particularly preferably within the range from 1 ppm to 1000 ppm. The vacuum dewatering column is then operated in such a way as to produce a top-of-column product consisting predominantly of water and having a monoethylene glycol content of below 5% by weight, preferably below 1% by weight, preferably below 1000 ppm, and a base-of-column product consisting predominantly of glycol and having a water content of below 5% by weight, preferably below 1% by weight, particularly preferably below 1000 ppm. The vacuum dewatering column has withdrawn from it a sidestream which is substantially free of monoethylene glycol, i.e., with a monoethylene glycol content of below 5% by weight, preferably below 1% by weight, particularly preferably below 1000 ppm, and enriched with specification-destructive secondary components, especially medium boilers and also low boilers. The dewatering column is operated with a base-of-column temperature of not more than 220xc2x0 C., preferably from 120xc2x0 C. to 200xc2x0 C., particularly preferably from 160xc2x0 C. to 180xc2x0 C.
The feed to the vacuum dewatering column is generally the base-of-column effluent from the pressure dewatering column or the last column of the pressure dewatering battery. In individual cases, however, it is also possible to feed the vacuum dewatering column directly with the effluent from an EO hydrolysis reactor. The base-of-column product of the vacuum dewatering column is substantially water-free and is fed to the monoethylene glycol purifying distillation. The top-of-column product, substantially monoethylene glycol-free water, is wholly or partly further used in the process, particularly fed to the hydrolysis reactor. The sidestream can be discharged into the wastewater or be further worked up.
In a further preferred embodiment, two vacuum dewatering columns are connected in series. The glycol-containing stream to be purified is fed to the first vacuum dewatering column. The base-of-column product of the first vacuum dewatering column is fed to a second vacuum dewatering column, preferably into the middle section thereof. Typical glycol concentrations in the bottom product of the first vacuum dewatering column are 70-99.5% by weight, preferably 85-99.5% by weight, particularly preferably 95-99% by weight. The head product withdrawn from the second vacuum dewatering column is an aqueous, substantially glycol-free stream which has a glycol content of below 5% by weight, preferably below 1% by weight, particularly preferably below 1000 ppm and is rich in medium boilers and also low boilers. The bottom product of the second vacuum dewatering column is substantially anhydrous glycol; it is fed to the monoethylene glycol purifying distillation. The base-of-column temperatures in the vacuum dewatering column(s) should generally not exceed 220xc2x0 C., preference being given to the range from 120xc2x0 C. to 200xc2x0 C. and particular preference to the range from 160xc2x0 C. to 180xc2x0 C.
It is particularly advantageous to supply the middle section of the only or last vacuum dewatering column with a overhead stream from the monoethylene glycol purifying distillation. This measure makes it possible to remove from the system even secondary components which are formed as a consequence of secondary reactions in the monoethylene glycol purifying distillation. The overhead stream is advantageously small, especially within the range from 1 to 10%, based on the pure monoethylene glycol stream. To minimize the overhead stream to be recycled, the secondary components in the overhead stream have to be concentrated. This requires additional separating stages between the point of removal of the pure monoethylene glycol (side takeoff) and the stream to be recycled; that is, some separating stages have to be disposed between the top-of-column takeoff and the monoethylene glycol sidetakeoff in the monoethylene glycol purifying distillation column, preferably from 1 to 10, particularly preferably from 3 to 6, separating stages. An advantageous side-effect of concentrating and recycling the secondary components is that the small amounts of water present in the column feed to the monoethylene glycol purifying distillation are returned into the vacuum dewatering. This provides a monoethylene glycol having an extremely low water content.
In a particularly advantageous version of the process, the removal of secondary components, especially low boilers, in the pressure dewatering stage is improved as well as the removal in the vacuum dewatering stage. To this end, the pressure dewatering column or at least the first pressure dewatering column of the battery has a stripping section with at least one separating stage, preferably with from 2 to 10 separating stages, particularly preferably with from 3 to 6 stages, and a portion of the overhead stream of the dewatering column(s) having a stripping section is removed from the system.
Conventional large industrial processes utilize an acetaldehyde purge at the bottoms reboiler of the second pressure dewatering column: this is where the vapors of the first pressure dewatering column are substantially condensed, with the uncondensed fraction, about 1-5% by weight of total vapors, being removed from the system. The remaining vapors may, if desired, be postcondensed in a further heat transferor, and the heat of condensation may be utilized at a suitable location in the overall process. However, this conventional solution will remove via the acetaldehyde purge only secondary components which leave the first pressure dewatering column as part of the vapors. This is inadequate in the case of formaldehyde in particular, since the volatility of formaldehyde in aqueous glycol solutions decreases with increasing glycol content, especially as a consequence of chemical reactions of the formaldehyde with water and glycols. So as to separate formaldehyde from the glycol-containing bottom product of the pressure dewatering column, the pressure dewatering column or at least the first pressure dewatering column of a battery requires a stripping section of at least one stage, preferably from 2 to 10 stages, particularly preferably from 3 to 6 stages. Only when the formaldehyde has been removed into the purely aqueous vapors of the first column can it be purged from the system together with acetaldehyde. The efficiency of removal of the formaldehyde in the stripping section improves with the temperature and correspondingly the pressure in the pressure dewatering column, or in the first pressure dewatering column of the battery, and with the water content of the reactor effluent. Two of the additional plates in the stripping section can be saved if the bottoms reboiler is constructed as a xe2x80x9cdivided basexe2x80x9d as described in DE-C-33 38 488.
The amount of secondary components, especially acetaldehyde or formaldehyde, removed from the system depends on the amount of wastewater removed. It has to be borne in mind, however, that the amount of vapor not condensed in the bottoms reboiler of the second dewatering column cannot be increased ad infinitum for reasons of the integrated energy system and on account of control-engineering restraints. The inventors have found a particularly preferred version of the process, whereby further removal of secondary components from the condensed vapor is possible by steam stripping. The stripping steam loaded with secondary components can subsequently be utilized for its energy content at a suitable location in the process. Steam stripping, therefore, requires no additional energy, only an additional apparatus. The removal of secondary components from the system is particularly effective when the effluent from the stripper is refluxed into the first dewatering column, since this recycling will increase the aldehyde content at the top of the first pressure dewatering column and in the stripper and hence also the removal rate.
Advantageously, the temperature below the feed point into the pressure dewatering column is above 80xc2x0 C., but preferably within the range from 100xc2x0 C. to 250xc2x0 C., particularly preferably within the range from 115xc2x0 C. to 230xc2x0 C., the pressure in the stripping section being not less than 1 bar, preferably within the range from 2 to 30 bar.
Advantageously, the overhead stream of the pressure dewatering column(s) having a stripping section is introduced into a partial condenser and/or a stripper, especially a steam stripper, and the gaseous stream(s) enriched with secondary components is (are) removed from the system.
Suitably, the partial condenser and/or the stripper are operated at above 90xc2x0 C., preferably at from 120xc2x0 C. to 250xc2x0 C.